North America has been experiencing some incredible weather of late. After a very mild winter in much of the continent, there has been record-breaking summer-like weather for over a week - in March! In Canada, for instance, the thermometer in St John hit 25.4 °C on 21 March, smashing the previous record high for March of 17.5°C.
"We've never seen these kind of temperatures before. It's quite remarkable," Dave Phillips, a senior climatologist at Environment Canada, told local media.
“The duration, areal size, and intensity of the Summer in March, 2012 heat wave are simply off-scale,” says Jeff Masters of the Weather Underground. “The event ranks as one of North America's most extraordinary weather events in recorded history.”
The "summer in March" is now coming to an end but what caused it? Meteorologists have been pointing to two main factors.
Firstly, as pointed out by Masters, a big loop in the jetstream developed over the continent, funnelling warm air from the Gulf of Mexico northwards. This loop in the jetstream remained "stuck" in place for over a week, a phenomenon known as a blocking pattern.
Secondly, as Phillips points out, air flowing northwards in spring would normally be cooled as it passes over cold, snowy ground. But this year there is very little snow on the ground because of the mild winter and the air is hardly cooling down at all.
So is there a link with global warming? Well, sea surface temperatures in the Gulf of Mexico have been much higher than normal over the past couple of months, which means the air flowing north would have been warmer otherwise to start with. Such anomalies are obviously becoming more common as the sea heats up.
Mild winters with little snow are also more likely in a warmer world. And global warming may even have influenced the jetstream pattern. According to research published last week by Jennifer Francis of Rutgers University (Geophysical Research Letters, vol 39, L06801), the rapid warming in the Arctic is affecting atmospheric circulation further south, making weather patterns more persistent - more blocking, in other words - which makes some kinds of extreme weather events such as heatwaves more likely.
Thinking about all this made me wonder whether there is a kind of "multiplier effect" at work. While many factors had to coincide to produce the kind of weather we've seen in the US and Canada recently, extreme warmth is clearly more likely in a warmer world. However, if the "summer in March" is the result of a combination of extreme weather events, and each of those individual events has been made more likely by global warming, then the link with global warming is much stronger than it appears at first.
I asked Kevin Trenberth, a climate scientist at National Center for Atmospheric Research in Boulder, Colorado, what he thought. "Yes I think you are right but it is hard to prove," he replied. He has just published a paper on climate extremes, which points out that while meteorologists tend to look at extreme events individually, many may be related.
Masters thinks it is plausible, too. “A climate model should be able to track all these synergistic effects and anticipate a multiplier effect like this, but they are not detailed enough yet to do this,” he says.
While this is all speculative, it is also rather worrying. If the 0.7 °C warming of Earth so far is indeed largely to blame for the unprecedented, record-shattering spring warmth in North America, what kind of extremes are we going to see later this century when the planet is 3 or 4 °C hotter?
